Prolonged activation of the mitogen-activated protein kinase pathway promotes DNA synthesis in primary hepatocytes from p21(Cip-1/WAF1)-null mice, but not in hepatocytes from p16(INK4a)-null mice

In primary rat hepatocytes, prolonged activation of the p42/44 mitogen-acti vated protein kinase (MAPK) pathway is associated with a decrease in DNA sy nthesis and increased expression of the cyclin-dependent kinase inhibitor ( CKI) proteins p21(Cip-1/WAF1) and p16(INK4a). To evaluate the relative impo rtance of these CKIs in mediating this response, we determined the impact o f prolonged MAPK activation on DNA synthesis in primary cultures of hepatoc ytes derived from mice embryonically deleted (null) for either p21(Cip-1/WA F1) or p16(INK4a). When MAPK was activated in wild-type mouse hepatocytes f or 24 h, via infection with a construct to express an inducible oestrogen r eceptor-Raf-1 fusion protein (Delta Raf: ER), the expression of p21(Cip-1/W AF1) and p16(INK1a) CKI proteins increased, cyclin-dependent kinase 2 (cdk2 ) and cdk4 activities decreased, and DNA synthesis decreased, Inhibition of RhoA GTPase function increased the basal expression of p21(Cip-1/WAF1) and p27(Kip-1) but not p16(INK4a), and enhanced the ability of MAPK signalling to decrease DNA synthesis. Ablation of the expression of CCAATT enhancer-b inding protein alpha (C/EBP alpha), but not of the expression of C/EBP beta , decreased the ability of MAPK signalling to induce p21(Cip-1/WAF1). When MAPK was activated in p16(INK4a)-null hepatocytes for 24 h, the expression of p21(Cip-1/WAF1) increased, cdk2 and cdk4 activities decreased and DNA sy nthesis decreased. In contrast with these findings, prolonged activation of the MAPK pathway in hepatocytes from p21(Cip-1/WAF1)-null mice enhanced cd k1 and cdk4 activities and caused a large increase in DNA synthesis, despit e elevated expression of p16(INK4a). Inhibition of RhoA GTPase activity in p21(Cip-1/WAF1)-null cells partly blunted both the basal levels of DNA synt hesis and the ability of prolonged MAPK signalling to increase DNA synthesi s. Expression of anti-sense p21(Cip-1/WAF1) in either wild-type or p16(INK4 a)-null hepatocytes decreased the ability of prolonged MAPK signalling to i ncrease the expression of P21(Cip-1/WAF1), and permitted MAPK signalling to increase both cdk2 and cdk4 activities and DNA synthesis. These results ar gue that the ability of prolonged MAPK signalling to inhibit DNA synthesis in hepatocytes requires the expression of p21(Cip-1/WAF1), and that the inc reased expression of p16(INK4a) has a smaller role in the ability of this s timulus to mediate growth arrest. Our results also suggest that RhoA functi on can modulate DNA synthesis in primary hepatocytes via the expression of p21(CiP-1/WAF1) and p27(Kip-1).